Quite regional.
In some areas (like mine) A lot of it is directly related to childcare costs.
In our area it's 1600 a month for infant care until the child is potty trained. For those who can actually afford 1600 a month (aka not be losing money working), the question is how much *more* do you need to be making after paying for childcare? $100 a month? $500 a month? $1000 a month? $2000 a month? (breaking it down hourly is even more depressing, when I *finally* would have been in the 'black' I would have been making $1.60 an hour. Um. $8 a day after taxes is just not worth it to me to work any job that wasn't a career. AKA if I was going to essentially be working for free I had better be 'moving up the ladder'. Any other jobby-job? Heck no.) If you're making $1-3 an hour after paying $10 an hour for childcare... is that worth it to you? Of course, for families with more than 1 child, those numbers alter a bit. $3200 a month for 2 kids in diapers, or $4000 a month for 2 kids in diapers and 1 out. But the cost benefit analysis is still mostly the same.
In our area, there is typically 1 SAHP until their youngest is out of diapers OR parents work opposite shifts. Because...
Another factor is housing costs. A 2b/1ba starter home in our area is half a million. Soooooo many people go back to work as soon as daycare costs halve (when the youngest is potty trained), so that they can get out of an apartment (average is 1200-1700 a month for a 2b/1ba apartment) and into a house with a yard (2500 a month and up).
In our area SAHPs after the kids start school are the minority. So much so that the public schools have 'Moms & Muffins' and 'Dads & Donuts' (hey! I wanna donut! Stupid 'healthy' muffins! Give me an eclair!) and other parent interactive things BEFORE school hours. Ugh. I had to drag myself up out of bed to be at school at 6am 4 times the year my son was in school. But having super early stuff (instead of the 10am stuff they did 20 years ago) is the only way to get parental involvement, because most parent work.
It's ALSO what's *AVAILABLE* to stay at home parents once their kids are in school that in part determines what's regionally normal (after daycare costs for wee ones, and housing costs in general). Some areas which are very PRO stay at home parent have tons of neighborhood and community things for stay at home parents. In other areas, you can feel like you're losing your mind of loneliness and boredom.
I was absolutely blown away visiting a friend in Utah. There was so much to do! All child friendly ADULT get togethers. BBQs, and "game nights" -at 10am, or 2pm), and swimming groups, and jogging groups, and singing, sports, reading, car repair... pick an INTEREST and there was a parent group for that that met during the school day!!! Kids welcome! An area set aside for naps and nursing! Neighbors actually got together to hangout. Mindblowing. Coming from an area where most families are dual income, and the only "activities" during the day are KID related (toddler dance, gymnastics, playdates, etc.) it actually floored me to see OUR brains being encouraged not to turn into oatmeal.
Same token, in our area SAHPs are (generally) looked down on. Families will pay a person or a facility several thousand a month for childcare, but that same respect (for working a highly paid job) just isn't given to stay at home parents. In other areas dual income parents are looked down upon. It very hard REGARDLESS of your choice to live in an area where you are 'looked down upon' by people in general.